S.F.'s concrete buildings also a quake danger

Quake retrofit triage plan gives wood structures top priority

Robert Selna, Chronicle Staff Writer

Published 4:00 am, Monday, July 28, 2008

San Francisco Mayor Gavin Newsom's recent push for an earthquake-retrofit study of some of the city's shaky wood-frame buildings means a plan for potentially deadly concrete structures is on hold for now.

Local engineers estimate that San Francisco has several hundred concrete-frame buildings that are liable to crumble in a big earthquake centered close to the city. The precarious structures come in different shapes and sizes - some are high-end condominiums on Nob Hill, and others are former printing houses South of Market. But they were all constructed before building code revisions in the mid-1970s that demanded more steel reinforcement in and around beams and columns, and stronger walls.

City officials have decided to speed an analysis of what to do about so-called soft-story buildings because the wood-frame structures are ubiquitous in San Francisco - possibly numbering in the tens of thousands - and probably represent a greater risk simply because of their numbers.

Soft-story buildings include the classic San Francisco apartment building with a store or restaurant on the first floor, as well as the Sunset District home built over a garage. They have a plate-glass window or garage door on the ground floor instead of a solid wall, making them susceptible to twisting and buckling.

The inflexible, or "nonductile," concrete buildings did not collapse in the 1989 Loma Prieta earthquake the way that Marina district soft-story apartment buildings did, but experts say they could pancake in a quake centered near the city, making it unlikely that people inside would survive.

"If you look at dangerous buildings, these are high on the list," said David Bonowitz, a structural engineer who sits on the city's seismic safety advisory committee. "They often are big and multistory, and when they collapse, people die."

Nonductile concrete buildings tend to collapse in big temblors because they don't have enough steel in their columns and beams to allow them to bend and twist. Instead, their supports crumble and even explode under too much pressure.

Dismal safety record

The buildings have a dismal earthquake safety record, collapsing in many U.S. quakes, including the 1971 San Fernando and 1994 Northridge temblors, and claiming thousands of lives overseas in Mexico, Japan and Turkey.

Oakland's former Cypress Structure was a nonductile concrete roadway. The 1 1/2-mile elevated freeway's top deck collapsed in Loma Prieta, killing 42 motorists and injuring hundreds more. Unstable fill soil at the freeway's base made its weak columns even more hazardous. The mangled roadway was torn down, and traffic was rerouted onto new structures.

On July 8, Newsom introduced legislation to address the more widespread threat posed by the soft-story buildings, which for decades had largely been ignored by city engineers and leaders.

Newsom's legislation would waive fees and expedite retrofit permits, and he has instructed the city's Department of Building Inspection to help create retrofit guidelines for soft-story buildings. Some Bay Area cities have required property owners to retrofit their soft-story buildings; Newsom's voluntary measure was seen as more politically agreeable than a mandatory plan.

Get in line

The building department has made soft-story buildings a top priority, directing consultants to analyze the risks and develop retrofit plans. But that means other vulnerable structures will have to wait, according to department Director Isam Hasenin. The consultants don't have the staff to simultaneously analyze each type of vulnerable building in San Francisco, he said.

The city hopes to make recommendations on how to retrofit soft-story buildings by the end of January, but devising a plan for other buildings is now more than 18 months away.

While soft-story retrofits are relatively cheap and easy, it is enormously expensive to identify susceptible concrete buildings and shore them up, according to Craig Comartin, director of a group of structural engineers called the Concrete Coalition.

The engineers are working to understand the scope of the concrete structures' danger and the best way to ameliorate it.

A retrofit for a nonductile concrete building requires infilling with new shear walls, "jacketing" or wrapping columns with steel rebar or carbon fiber, or constructing steel braces - or some combination of all three.

Comartin, who has advised UC Berkeley's retrofit of 12 concrete buildings over the past 10 years, estimated that typical strengthening jobs cost $20 to $150 per square foot of floor space. In some instances, repairs can total 60 to 70 percent of the cost of replacing the building, he said.

'It's not cheap'

"It's not always clear from the outside that there is glaring deficiency," Comartin said. "Sometimes it takes an engineer to really go through and determine whether or not the building is dangerous. It's not cheap."

The high repair cost explains why no big California city has approved a law requiring retrofits of nonductile concrete buildings. The Los Angeles City Council tried in 1995, but resistance from building owners killed the plan.

Greig Smith, an aide for a Los Angeles city leader in 1995 and now a city councilman whose district includes Northridge, is working with Comartin to develop an inventory of that city's nonductile buildings.

Smith said he intends to introduce legislation in the coming months that will set dates by which owners of the dangerous buildings must retrofit their property. In the meantime, he said, he's working on incentives, such as using public bonds to provide loans to owners who retrofit voluntarily.

"This is not easy to do because it's so expensive," Smith said. "Last time we talked about this, I got a lot of phone calls from building owners saying, 'Whoa, wait a minute.' But it's something that needs to be done."

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